Comparison of elasticity patterns of elasmobranchs and mammals with review of vital parameters of lamnids
Henry F. Mollet, Moss Landing Marine Labs and Monterey Bay Aquarium:
Vital parameters (age-at-first-reproduction, maximum-reproductive-age, age-specific fertilities, and age-specific mortalities) of lamnids were reviewed. Despite progress in the last 20 years, vital parameters of lamnids and many other elasmobranchs are not sufficiently well-known to produce reasonably accurate population growth rates (lambda). Luckily, the elasticity pattern of a species that is needed to evaluate management proposals is fairly robust and does not require an accurate:lambda. Furthermore, there is no need to calculate lambda because the elasticity pattern is determined by age-at-first-reproduction (alpha) and generation time (Abar) alone, with gestation period (GP) providing a refinement: E(fertility) = E1 = 1/Abar; E(juvenile survival) = E2 = (alpha - GP) E1; E(adult survival) = E3 = 1 - E2 = (Abar - alpha + GP) E1. These are lower-level elasticities of the vital parameters as they appear in a life history table and E2 includes survival to age one. Therefore, they do not sum to one but are easily normalized. The exclusion of survival to age one from E2 distorts the elasticity pattern, in particular for species with low age-at-first-reproduction (alpha < 2). Abar is usually not known. However, using an Abar estimate based on the mean Abar/alpha ratio of 60 elasmobranchs (1.31, coefficient of variation 9.3%, range 1.1-1.8) provided promising results. The elasticity pattern of elasmobranchs as a function of age-at-first-reproduction indicated that E2 is largest for all elasmobranchs if alpha > 1 yr (valid for most if not all elasmobranchs). Accordingly, protection of juveniles will provide the most effective measure to reverse population declines if that has been observed for an elasmobranch species. The mean Abar/alpha ratio of 50 mammals (2.44, coefficient of variation 33.5%, range 1.2-5.0) is larger and more variable compared to that of elasmobranchs and, accordingly, their elasticity patterns are more complicated.